Re: ATM First project ( (NOT even a) Bit off too much? )

[Anthony Stillman <atmer@flash.net>]

Kristopher, all,

>I had started a 6" f4 last year, but got hung up trying to parabolize it...

Yes, f/4 is kinda fast for a first mirror.  Especially at 6"

>6" f4 Lurie-Houghten

Houghtens are nice instruments.  At the last Riverside, Gerry Logan brought
his 8".  With its eleven element mil-spec low power eyepiece, it provided a
beautiful 3 degree field which was crystal clear from edge to center.
After coveting it for the better part of an hour I opined to Mark V. that
it kind of made ATMing by anyone other than Gerry Logan, pointless.  Of
course, it hasn't stoped me, or anyone else I know who druled over it.

For those who don't know Gerry, he's a master optician, a very nice man who
cares for his friends, and is wonderfully humble.  While I sat with the
rest of his entourage, everyone who stopped by remarked what a gorgeous
piece of workmenship it was, both optically and mechanically.  You see,
Gerry mechined the mount, drive and the scope's metal parts.  Each time,
Gerry said something to the effect of, "Oh that, its nothing, it was easy",
in his Scottish accent, of course.

Well, I don't want to diswade you from attempting this telescope, but
frankly I wouldn't suggest it as first project.  I do want to point out
some areas you should be concern with.

First, the tolerances on radii are tight.  Its not as bad as a triplet, but
you will need to buy or construct some accurate instruments.

Second, you will need to concern yourself with wedge and center thickness
in both corrector elements.  Spacing, of course, is sort of a variable.

Lastly, you'll want a smooth finish on both correctors elements.  BAK-7 or
BK-7, isn't really that dificult to get a good polish on, but its more
dificult than pyrex.

Also, when figuring the corrector spheres, yes you'll have to figure, a
good sphere isn't automatic, you will need to allow the BAK-7 to
thermolize.  But,this just adds time to the process.

All in all I would suggest you try a sub-diameter lap or star lap and
parabolize the f/4.  Remember, the center of the mirror is shadowed so a
hole or a hill at the center can be ignored.

Anthony


Anthony